Liquid scintillation counters are commonly used to measure the count rate or activity of samples containing low energy beta particles or corresponding particles emitting radionuclides such as tritium and carbon-14.
The range of the low energy beta particles in the sample is generally few tens of micrometers at the most. As a consequence, the sample to be measured has to be placed in direct contact with a scintillation medium, which comprises a solvent or solvents and a solute or solutes present in a few percent by weight of the solution. In this interaction process most of the kinetic energy of the interacted beta particle is absorbed by the solvent and then transferred to the solute which emits scintillation photons, whose amount is proportional to the energy of the interacted beta particle. These scintillation photons are detected usually by two, in coincidence operating, photomultiplier tubes producing electric pulses. The height of the pulses are proportional to the amount of emitted scintillation photons and thus proportional to the energy of the interacted beta particle.
When measuring sample activities with liquid scintillation counters, the basic problem is the reduction of the counting efficiency due to the quenching of the sample.
It is known in the liquid scintillation counting that the reduction of the counting efficiency due to the quenching of the sample can be corrected by a means of a quench curve which describes the relationship between the counting efficiency and the amount of the quench of the sample.
Normally liquid scintillation counters are provided with one detector and they are designed to measure samples in 7 ml or 20 ml glass or plastic vials.
1450 MicroBeta, which is manufactured by Wallac Oy, Finland, is a novel example of a multi-detector liquid scintillation counter, which measures samples in sample plates, which belong to a preselected assortment of different multi-well sample plate types, each having a characteristic two dimensional array of sample wells.
One group of the multi-well sample plates belonging to the preselected assortment consists of 96-well sample plates having eight rows of wells, which diameter is 7-8 millimeters arranged in twelve columns with 9 millimeters distance between the center points of the wells. The typical volumes of the sample wells of such 96-well sample plates are 200-400 microliters depending on the height of the plate.
When the wells of the sample plate are separate, it is placed before counting on a rigid sample plate holder made of photon attenuating material and having thru-holes for the wells of the sample plate. The walls of the holes are reflecting or scattering in order to guide the photons from the liquid scintillation sample to the detectors, built of two photomultiplier tubes situated on the opposite sides of the holes of the sample plate holder. As a consequence, an optically isolated compartment is formed around each sample well of the sample plate.
If the wells of the sample plates are not separate for example sample wells are joined together with ribs in order to stiffen said sample plate, then it is impossible to isolate the wells optically. As a consequence of this some amount of the scintillation photons produced by the absorption of the beta particle in a certain sample well may travel to the nearest sample wells and thus produce an increase in observed count rates in those wells. This affection is called here as crosstalk. This phenomenon is a problem particularly when higher energy isotopes are measured: the higher the kinetic energy of the beta particles, the higher amount photons are produced in the interaction process and the higher amount photons can travel to the other sample wells.
U.S. Pat. No. 4 348 588 (Yrjonen et al.) shows a crosstalk correction method applied in gamma counting. This method is not suitable for liquid scintillation counting, because it does not take into account the variation of the quench of the samples, and it is limited to correct only affection of samples, which are in pre-determined locations.